A fuel cell is a power system having a cell constituted of a porous anode, a cathode and a dense-structured electrolyte as a base, ultimately producing water while ions migrate through the electrolyte when hydrogen is injected into the anode and air is injected into the cathode. Herein, electrons flow to the outside through a separator, and such a combination of the separator and the cell is referred to as a unit cell, while a plurality of such unit cells connected in series is referred to as a fuel cell stack.
More specifically, the unit cell is constituted of a separator, a cell and a current collector, and among these, the cell has a difference in constitution, depending on the fuel cell types such as PEMFC, MCFC and SOFC.
As an example, a solid oxide fuel cell (SOFC) has a cell structure formed of an anode, a cathode and an electrolyte. Herein, the anode, the cathode and the electrolyte are all formed of ceramic materials, and since these are laminated and then sintered at high temperature to be prepared as a cell of one sheet, the cell surface may not be flat and may have a certain level of surface roughness.
In a cell structure formed to have an anode, a cathode and an electrolyte as described above, a separator is used to electrically connect unit cells as well as to separate hydrogen and air used as fuel and to form a gas flow channel. Such a separator may be formed to be attached to an anode or a cathode through mechanical processing, etching, stamping or the like.
However, in forming the separator as above, a height difference may inevitably occur between the separator channels, since the anode or the cathode may not be flat.
Meanwhile, by further providing a current collector between the anode and the separator, and between the cathode and separator, the electrode and the separator may be in more electrically uniform contact with each other.
In a SOFC as an example, an Ni foam having a single composition is used as an anode current collector, and since the Ni foam still maintains metallicity in a reducing atmosphere under which hydrogen, a fuel, flows, no problems are caused in current collection. However, when a metal mesh or a metal foam is used as a cathode current collector, there may be a problem in that, with a cathode having an operating temperature of approximately 700° C. to 800° C. and having air flowing therethrough, metal materials of the cathode current collector may be quickly oxidized, losing current collecting efficiency.
In order to prevent such a problem, conductive ceramic pastes are normally used to form a cathode current collector. However, there are limitations to controlling the thickness of conductive ceramics when prepared through a process such as screen printing or powder spraying, which leads to limitations in securing current collecting areas while sufficiently reducing a height difference of a separator and reducing surface roughness of a cell.
As technologies disclosed in the related art, Patent Document 1 uses a metal oxide foam as a cathode current collector; however, this is a foam entirely formed in an oxide state from an initial installation stage, and therefore, has almost no capability to reduce a height difference of a separator or to reduce surface roughness of a cell, and in terms of a preparation method, preparing the collector to have a uniform thickness is difficult, since a method of coating a metal oxide slurry on a polymer is used. There is also a limit in that the composition is limited to only a perovskite structure.    (Patent Document 1) Korean Patent No. 10-0797048